Alcohols, such as saturated monohydric alcohols, are important chemical commodities which may be used as fuels or as components of fuels, as solvents, and as feedstocks for the preparation of other important commodity chemicals. Ethanol in particular is widely used commercially as a gasoline additive or as a fuel per se, as a solvent, as a germicide, as an antifreeze, as a component in the food and beverage industry, and as a chemical feedstock.
Ethanol, and to a lesser extent propanol and butanol, are of increasing significance as chemical feedstocks, since they are readily obtainable from biological sources, in particular by the fermentation of sugars and/or biomass. Alcohols from biological sources, so-called bio-alcohols, thus provide one way of reducing the dependence on crude oils for fuel uses and as chemical feedstocks.
Bio-alcohols, and bio-ethanol in particular, are typically produced by fermentation processes performed on biomass and/or derivatives thereof. As used herein, the term “biomass” includes sugar sources, for instance sugar beet, sugar cane, molasses and corn syrup, starches and cellulosic materials, such as lignocellulose. Starches and cellulosic materials are generally converted by enzymatic or chemical hydrolysis to produce simple sugars which can then be converted to bio-alcohols by fermentation. Ethanol obtained from cellulosic materials is commonly referred to as cellulosic ethanol or lignocellulosic ethanol.
Alcohols obtained by fermentation contain low levels of nitrogen-containing contaminants. One possible source of nitrogen-containing contaminants may be ammonia which may be introduced during the fermentation stage. Once in the process, the ammonia can react with ethanol and other impurities to form a variety of nitrogen-containing compounds. Nitrogen-containing contaminants are also often present in alcohols from other sources.
The presence of nitrogen-containing contaminants in alcohol compositions is undesirable since these compounds may interfere with subsequent chemical processing in which the alcohol composition is used as a feedstock. For example, nitrogen-containing contaminants can poison, deactivate or otherwise interfere with a number of catalysts which may be used in the processing of alcohol feedstocks, for example by neutralising acidic sites on heterogeneous acidic catalysts.
An example of the use of heterogeneous acidic catalysts for the processing of alcohols is the dehydration of alcohols to form olefins. Olefins, such as ethylene, have historically been produced by steam or catalytic cracking of hydrocarbons derived from crude oil. However, as crude oil is a finite resource, methods for the preparation of olefins by the dehydration of alcohols have been proposed. For instance, WO 2009/098262 discloses a process for the catalytic dehydration of an alcohol to the corresponding olefin wherein the catalyst is selected from a crystalline silicate, a dealuminated crystalline silicate or a phosphorus modified zeolite; WO 2008/138775 discloses a process for the dehydration of one or more alcohols comprising contacting one or more alcohols with a supported heteropolyacid catalyst in the presence of one or more ethers; and WO 2008/062157 discloses a heteropolyacid catalyst and the use thereof in a process for the production of olefins from oxygenates.
Olefins produced in this way have a range of potential applications, for instance as feedstocks for the production of polymeric materials. In particular, ethylene obtained by the dehydration of ethanol may usefully be processed into polyethylene. Similarly, the dehydration of propanols provides a route to propylene which may subsequently be processed into polypropylene.
It has been observed that catalysts used for the dehydration of alcohols, such as crystalline silicate, dealuminated crystalline silicate, phosphorus modified zeolite or supported heteropolyacid catalysts, are sensitive to the presence of low levels of nitrogen-containing contaminants in alcohol feedstocks. Consequently, in order to ensure commercially acceptable catalyst performance and lifetime, it is highly desirable to treat alcohol feedstocks to remove nitrogen-containing contaminants prior to the dehydration reaction. As noted above, the presence of low levels of nitrogen-containing contaminants is a feature of alcohol compositions obtained from at least biological sources.
The use of cation exchange resins and sorbents for the removal of nitrogen-containing compounds from hydrocarbon streams is known in the petrochemical industry. However, the sensitivity of many acidic catalysts, such as the supported heteropolyacid catalysts mentioned above, to nitrogen-containing contaminants is such that very stringent removal of these compounds is required, for instance to 1 ppm or less, preferably 0.5 ppm or less, and most preferably 0.2 ppm or less; very low concentrations of nitrogen-containing contaminants are desirable in order to obtain a useful catalyst lifetime. It has been observed that the use of cation exchange resins alone may be inadequate to obtain such stringent removal of nitrogen-containing contaminants from alcohol compositions and that additional treatment of the alcohol composition may be required. It is therefore of significant commercial interest to be able to identify new processes which are able to remove nitrogen-containing contaminants from alcohols, particularly bio-alcohols such as bio-ethanol.
WO 1997/045392 discloses a process for the production of ethers in which deactivation of an acidic ion-exchange resin etherification catalyst is reduced by separating nitriles from an olefin feedstock by aqueous extraction. The nitriles are subsequently separated into an alcohol phase and hydrogenated to form amines which are more easily separable from the alcohol phase by fractionation.
EP 1 176 132 A1 discloses a process for preparing ethers comprising reacting an alcohol and an olefin in the presence of an acidic catalyst. Excess alcohol is recycled to the reaction zone together with nitrile compounds originating from the olefin feed. To avoid accumulation of nitriles in the system and deactivation of the catalyst, the excess alcohol comprising nitrile compounds is contacted in the liquid phase with a solid acid prior to being recycled to the reaction zone. It is reported that this reduces the level of nitriles in the recycled alcohol stream by at least 50%.
U.S. Pat. No. 6,770,790 discloses a process for removing oxygen-containing impurities from tertiary butyl alcohol comprising contacting the tertiary butyl alcohol in the liquid phase with at least two solid adsorbents, wherein the at least two solid adsorbents comprise aluminium oxide and a large pore zeolite.
WO 2010/060981 discloses a process for the purification of an alcohol in the course of a process for the preparation of olefins by acid-catalysed dehydration of the alcohol, the process comprising contacting the alcohol with one or more adsorbent materials. It is disclosed in WO 2010/060981 that while ammonia and amines can be adsorbed, nitrile impurities such as acetonitriles must be hydrogenated to provide modified impurities which are more readily adsorbed. Thus, according to WO 2010/060981, the alcohol feed is subjected to a hydrogenation step prior to contacting the alcohol with the one or more adsorbent materials. The Examples of WO 2010/060981 teach the removal of basic compounds from bio-ethanol by adsorption on a sulfonic acid resin at ambient temperature and pressure.